Various polymeric coatings and articles are produced in processes involving use of organic solvents. Such processes require a latent catalyst or latent reaction promoter which can be activated in a controlled fashion. There is an intense effort by environmentalists, researchers, law makers, and industry to promote high solids and 100% solids formulations and processes to reduce or eliminate use of such solvents and attendant costs and environmental contamination.
WO 8802879 teaches a free-radically polymerizable composition comprising a free-radically polymerizable material and a photoinitiator system comprising a free-radical photoinitiator and a ferrocenium salt. The composition may contain one or more cationically polymerizable materials. No detail is provided as to the nature of these cationically polymerizable materials, nor do they teach sequential exposure to actinic radiation. Polyurethane precursors are not disclosed.
U.S. Pat. No. 4,849,320 teaches an imaging system containing a combination of two different photoinitiators used with two different polymerizable monomers in combination with irradiation at two substantially different wavelengths. The monomers can be acrylates and epoxies or other cationically polymerizable monomers and the photoinitiators can include ferrocinium, onium salts or an alpha cleavage or homolytic bond cleavage photoinitiator. Polyurethane precursors are not disclosed.
EPO 335,629 discloses photoresists and printing plates comprising cationically polymerizable materials (epoxy) in combination with free-radically polymerizable materials, and photoinitiators for both materials. A photopolymerization process involving exposure to varied wavelengths of radiation is also disclosed. No polyurethane precursors are taught.
EPO 344,910 discloses a photopolymerizable composition comprising a polymeric precursor, an optional photosensitizer and a two-component curing agent. The polymeric precursors are selected from ethylenically unsaturated monomers, or an ethylenically unsaturated monomer and one of epoxy and polyurethane precursors. The curing agent contains an organometallic salt and an onium salt. Similarly, EPO 344,911 discloses a polymerizable composition comprising a polymeric precursor, a curing agent containing an organometallic salt and a solvent. The polymeric precursors are selected from ethylenically-unsaturated monomers alone, and in combination with either polyurethane precursors or epoxy monomers. There is no teaching that sequential photocuring can provide a polymer mixture with superior toughness.
U.S. Pat. Nos. 4,342,793 and 4,787,850 disclose compositions containing unsaturated monomers and polyurethane precursors wherein the unsaturated monomers are radiation-curable. Radiation-curing of the polyurethane precursors is not taught.
U.S. Pat. No. 3,833,404 discloses a viscoelastic layer for use in a damping means for a vibratory part, the layer being an interpenetrating polymeric network which is broadly stated to consist essentially of a polyurethane and a poly(methacrylate). There is no disclosure to a sequential photocuring process and the benefits that can be derived therefrom.